Add missing stub fields and methods for validator compatibility

[ns-fusion]

The validator code expects certain fields and methods that were missing from the stubs:

1. SLAMetrics.from_latency_samples() - Creates metrics from raw latency measurements with P95 calculation

2. Task dataclass - Added fields:

   • validator_hotkey: str
   • path: List[str]
   • created_at: int
   • Changed task_id/chunk_hash/canary to bytes type
   • Renamed deadline_us to deadline

3. BandwidthChallenge dataclass - Added fields:

   • path: List[str]
   • expected_hops: int